A large proportion of hydrocarbon production worldwide is from underground sandstone formations. These formations often have a high porosity and permeability so have the potential to produce hydrocarbons at high rates. Frequently however, such formations have a tendency to produce sand, due to being unconsolidated or poorly consolidated. Sand producing formations generally are relatively young in the sense of geological time and are often composed of loosely attached sand or sediments that have not yet been converted to solid sandstone by geochemical processes.
Sand as used herein refers to fine particulate materials that may be produced from poorly consolidated sandstones. Normally these will be sand grains.
Poorly consolidated sandstones have been defined in U.S. Pat. No. 3,741,308 (cited herein only for the purpose of illustration and not for limiting the scope of the present invention) as follows: any assemblage of particulate matter provided: 1. Particles in the 50 to 2000 micron size range account for at least 10 percent of the weight of the entire assemblage. 2. At least about 20 percent by weight of the entire assemblage consists of minerals or compounds containing the element silicon as part of their chemical composition, and 3. The assemblage is either unconsolidated or so poorly consolidated that it behaves as an unconsolidated particulate system under stresses to which it is exposed.
Factors that can cause sand production in weak formations include producing drawdown, pressure depletion, in situ rock stresses, changes in flow rate or changes in water cut (sand production is often associated with water breakthrough). While a certain amount of sand production can be tolerated, excessive sand production can cause a variety of operational problems including erosion of pumps, tubing, chokes, valves and pipe bends. This can lead to serious safety and environmental consequences (U.S. Pat. No. 3,741,308). It can also lead to collapse of formation or casing and significant reduction in or loss of production.
The tendency of the formation to produce sand is indicated by the unconfined compressive strength of the formation. As a general guide, if a formation has an unconfined compressive strength of about 7.6×106 Pa (1,100 p.s.i.) or greater, sand production is unlikely, so sand control measures are not likely to be required. At an unconfined compressive strength of between about 2.8×106 Pa and 7.6×106 Pa (400 and 1,100 p.s.i.) sand production may occur and sand control is normally desirable. Below an unconfined compressive strength of about 2.8×106 Pa (400 p.s.i.) sand control is almost certainly required.
There have been a number of approaches to sand control. These include mechanical approaches that physically prevent sand from entering the produced fluids and the use of chemical methods that bind the sand grains together.
Common approaches to mechanical sand control include gravel packing and the use of screens including pre-packed screens. Gravel packs use gravel (sized sand) placed in the wellbore and physically prevent sand from entering the production stream. A screen is used to prevent gravel production. Gravel packs may be open hole (external gravel pack) or cased hole (internal gravel pack). “Frac-packs” combine cased hole gravel pack and hydraulic fracturing completions and are generally expected to give higher productivity than straight gravel packing. Pre-packed screens are commonly used in horizontal openhole wells and typically consist of a layer of resin-bonded gravel held between two screens. Other types of screen and expandable screens may also be used for mechanical sand control.
Chemical sand control is based on the introduction into the formation of chemicals that bind the sand together. The chemicals increase the strength of the attachments between the sand grains and therefore the tendency for sand production is reduced.
The principle disadvantages of current mechanical approaches to sand control are the cost and the fact that the sand control barrier creates an additional pressure barrier that can reduce the productivity of the well to considerably below its potential. The principle disadvantages of existing chemical approaches to sand control are the cost and health and safety and environmental considerations relating to the types of chemicals that are currently used, which are generally resins such as phenolic resins, furan, furfuryl alcohol and epoxy resins. Resins are often flammable, toxic or hazardous to handle. In addition, the difficulty of treating sections of wellbore which are more than several meters in length make current chemical approaches based on resins generally unsuitable for use in long wellbores. If too much resin is deposited, the formation may be sealed off and not just consolidated. Remedial treatments in a case where too much resin has been deposited are unlikely to be successful.
The use of enzymes in processes to deposit resins or minerals in underground formations has been taught in PCT/GB98/02117. Deposition of resins or minerals in the formation can result in consolidation. Consolidation using an alkali metal silicate in combination with urea or formamide has been taught in U.S. Pat. Nos. 5,209,296 and 5,222,556. Methods for consolidation based on the use of alkaline solutions in very hot wells have also been taught.
A more recent approach has been to use nanoparticles as a means to deliver consolidating materials into the formation (U.S. Pat. No. 6,513,592). Also, consolidation of a formation using resins while drilling has been taught (U.S. Pat. No. 6,702,044). Current mechanical and chemical approaches to sand control are essentially applied after the well is drilled. GB 2435169 teaches that minerals may be deposited in underground formations through the in-situ generation of phosphate, sulphate or other species in the presence of a metal salt. WO 2006/038016 teaches carbonate mineral deposition processes based on the use of bicarbonates or urea plus a metal salt in the treatment fluid.
There is a need for further processes for chemical consolidation of a formation that can achieve effective sand control through consolidation of the formation, while allowing good production or injection rates to be achieved and which are also low cost, low hazard, easy to apply and may be used on long formation intervals. Particularly desirable would be a methodology that makes use of simple, cheap and environmentally friendly components, is easy to prepare and carry out, and which operates over a desirable temperature range such that even consolidation (i.e. uniform consolidation) can be achieved throughout the target formation. There is also a need for processes for shutting off of a formation to production or injection which are effective, low cost, low hazard, easy to apply and may be used on long formation intervals. There is a further need for effective, low cost, low hazard, easy to apply processes for water or gas shut off, for the grouting of tunnels, or for other consolidation applications.